DESIGN NOTE RIPRAP-LINED OUTLETS FOR PIPES DESIGN PROCEDURE ________________________ Land Owner ________________Service Center _________________________ County ________________ Date 1/6 WVENG_DN_RLOP 04/14 Outlet protection as presented here is a level apron of sufficient length and flare such that the expanding flow (from a pipe or culvert to a channel) losses sufficient velocity and energy that it will not erode the downstream channel reach it is discharging into. The design curves are based on circular conduits flowing full and the curves provide the recommended apron size and riprap dimension based on a minimum d 50 size. Figure 1a present curves for a low or minimum tailwater condition and Figure 1b presents curves for high or maximum tailwater condition. This design procedure is applicable to all pipe, culverts and storm sewer outlets greater than six inches in diameter and less than 48 inches in diameter and for water detention structure outlet pipes 24 inches (60 cm) in diameter or less. For detention structures having outlets pipes greater than 24 inches in diameter or general outlet pipes or culverts greater than 48 inches in diameter, energy dissipating outlet structures such as a Riprap Lined Plunge Pool for Cantilever Outlet (DN6) or those discussed in Technical Releases (TR) such as TR 54 Structural Design of F Stilling Basins, Saint Anthony Falls (SAF) Stilling Basin NEH, Section 14, Chute Spillways TR 50 “Design of Rectangular Structural Channels” may be used, as appropriate. DESIGN PROCEDURE STEPS Document the principal pipe diameter (inches), discharge (cfs) of the culvert conduit when in full flow condition (based on the inlet and outlet flow elevations). 1. Based on the tailwater depth immediately below the pipe outlet classify the tailwater condition as, a. minimum tailwater condition (Figure 1a) where the tailwater depth is less than half the diameter of the pipe, at the discharge point. b. maximum tailwater condition (Figure 1b) where the tailwater depth is equal or greater than half the diameter of the pipe, at the discharge point. 2. Select Figure 1a or Figure 1b for the appropriate tailwater condition. Determine the a. median d 50 stone diameter. i. Go to the selected figure and select the full pipe discharge (cfs) (x axis), extend a vertical line to the designed pipe diameter (in) to the lower set of curves. Then read horizontally to the y axis on the right and determine the median stone d 50 diameter (ft.). b. minimum length of Apron (La) in feet. i. Extend the vertical line (discussed in 2ai) to the top group of curves in the figure until it intersects the design pipe diameter. Then read horizontally to the y the axis on the left and determine the minimum apron length.